Rate and attitude indicating gyroscopic apparatus is described, for example, in U.S. Pat. No. 2,309,853, which apparatus is intended to provide navigational information, such as the heading of an aircraft with respect to the meridiam or other reference, the rate of turn of the aircraft, and the angular attitude of the aircraft about its longitudinal and transverse axes with respect to a particular reference position. The apparatus described in the patent employs a vibratory rod or wire which is secured at one or both ends for providing rate and attitude indications. Suitable electrical pickoffs are provided adjacent the vibratory element which serve to produce electric signals which are introduced to appropriate indicators. The drive of the vibratory element is generally controlled by one of two means, namely, by a "parametric drive" or by a "velocity feedback drive".
In a typical parametric drive, the bending motion of the vibratory element is sensed, and a resulting axial force is applied to the element. The resulting increase in tension tends to straighten the element and in the process increases its linear momentum. This makes up for the momentum lost in normal dissipative reactions and thereby maintains the system in vibration. For the parametric drive system to be operative, the force must be applied when the velocity of the vibratory element is towards the center, and since that condition occurs twice for each vibratory cycle, the force must be applied at twice the frequency of vibration. This type of drive permits the vibratory element to vibrate in any direction, which is a desirable characteristic in the vibratory type of gyroscope. However, the parametric drive is relatively inefficient, and substantial forces must be applied to the vibratory element to maintain it in its oscillating condition.
A more efficient drive means for the vibratory element is to provide movement-sensing and force-applying means in the form of capacitor electrodes which are mounted so that linear movements of the center of an end-supported elongated vibratory element may be sensed in each of two orthogonal directions. The signals generated by the sensing electrodes are amplified, processed and compared with reference signals, and the resulting differential signals are further processed to obtain electrical signals that are applied to the force-applying capacitor electrodes in order that the orthogonal movements of the center of the vibratory element may be maintained constant. Since the force is applied directly to the element in the velocity feedback drive type of system, the drive realized by this type of system is considerably more efficient than the drive provided in the parametric drive system.
In the type of end-supported elongated vibratory element system under consideration, a principal object is to maintain the plane of vibration of the elongated vibratory element fixed in inertial space so that angular displacement of the element with respect to inertial space may be measured. In general, however, due to minute imperfections in the physical structure of the element, the natural frequency of the element is not constant for all directions of vibration. Therefore, unless compensation is made, errors will arise in this type of system.
The resultant vibration of the center of the end-supported vibratory element may be regarded as the sum of two vibrations in the direction of the two principal axes. If the vibration frequencies along these two axes are not identical, a phase shift will develop which forces the orbit of the center of the vibratory element to become elliptical. The axis of the resulting ellipse may oscillate back and forth through substantial angles up to 90.degree. , thus impairing the accuracy of the instrument. This can lead to excessive drift rates, and this drawback is the principal reason why the prior art instruments of this type have not become generally accepted as precision navigational instruments.
The apparatus of the present invention, however, is constructed so that as the orbit of the center of the end-supported elongated vibratory element tends to become elliptical, the elliptical configuration of the orbit, and the drift rates resulting therefrom, are automatically suppressed. This is in contradistinction to the usual prior art parametric and velocity feedback drive systems which normally act to maintain the elliptical motion. However, all the advantages of the prior art parametric drive and velocity feedback drive systems are retained, in that the vibratory element is free to vibrate in any direction, the drive of the vibratory element is achieved with a high degree of efficiency, and the vibration amplitude of the vibratory element is maintained constant.